So how did Taylor Swift end up collaborating with Bon Iver on the track? Aaron Dessner revealed to Pitchfork
that William Bowery came up with the original demo, then Swift took his idea and ran with it. She sent it to The National band member as a voice memo singing both the male and female parts. Swift and Dessner discussed her potential duet partners and she told him how much she adores Justin Vernon's voice, and that he would "be so perfect."
Dessner and Vernon are good friends, so he told Swift he'd ask him, but with the warning, "It depends on if he's inspired by the song but I know he thinks you're rad."
Vernon did dig the song and wanted to contribute. As well as laying down his vocals, the Bon Iver mainman added the "so step right out" lyric to the bridge, a bit at the end and his choral parts.